trigger a system, when voltage is absent?

Discussion in 'The Projects Forum' started by haifa34, Aug 30, 2008.

  1. haifa34

    Thread Starter New Member

    Mar 24, 2008
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    Hi!

    well, I was looking for some kind of trigger system.

    that is my situation,

    I have a usb powered device, 5v and ground, obviously.
    When my device is off, I have an additional line of 3,3v. when my device is ON, i have a line with 0v

    Could I use this 3,3v-0v line to activate, deactivate my device? when the additional line hit 0v, the device should be activated, and when the additional line hits 3.3, the device must be turned off

    any suggestions?

    thanks!
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Can you give a better a explanation and how many wires do you have and what voltage each wire has?
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Do you know the current consumption of the 5V device?
     
    Last edited: Aug 30, 2008
  4. haifa34

    Thread Starter New Member

    Mar 24, 2008
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    well, its simple a wireless usb device, I think it will take less than 500mA

    I have a complete usb connection (5v,+data, -data, ground) and an additional line (one wire) that when the wireless device is connected, goes 0v, and when is disconnected, goes 3.3v.

    I will like to use that 3.3-0v line to trigger wireless activation, so I can leave the device always connected, and activate only when the additional line goes 0v ( I can control this additional line with one button)

    So I have to make a circuit that when 0v is present, let the 5v line be active, and when the 3,3v are present, cut the 5v line, or maybe we can use ground instead of 5v? Its the same for me

    thanks for reading, and sorry for my english, is not my language :)

    EDIT, looking at the wireless datasheet, it takes 50-200mA
     
    Last edited: Aug 30, 2008
  5. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    This should do the trick
     
  6. haifa34

    Thread Starter New Member

    Mar 24, 2008
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    thanks blocco, sounds good, I´ll test and report

    what is the comsumption when input voltage is 3,3v? something like 0.5mA, right?
     
  7. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Well, when the 3.3V is present, about 10mA is drawn from the 5V supply. This is the current that drives the base of the output transistor when the 3.3V is not present.

    You can halve the current by increasing the 470 Ohm resistor to 1k and this should be Ok as well. You just need to make sure that the output transistor is fully on under maximum load.

    There are other ways to reduce the current drawn from the 5V supply when the 3.3V is present, if necessary.
     
  8. haifa34

    Thread Starter New Member

    Mar 24, 2008
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    well, its for a portable device, any power consumption reduction, will be good, but I think that 10mA aren´t so important

    Ill try to find a protoboard around here, some transistor, and report
     
  9. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    The addition of another transistor will significantly reduce the off current:
     
    Last edited: Sep 2, 2008
  10. haifa34

    Thread Starter New Member

    Mar 24, 2008
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    thanks blocco, Ill try both diagrams and report
     
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